This invention relates to the processing of silver halide photographic elements, and more specifically to the desilvering of such elements.
During processing of color silver halide elements the silver is oxidized to a silver salt by a bleaching agent, most commonly an iron-complex salt of an aminopolycarboxylic acid, such as the ferric ammonium complex salt of ethylenediaminetetraacetic acid. The bleaching step is followed by removal of this silver salt and any unused silver halide by a fixing agent, such as thiosulfate, which renders the silver salts and silver halide soluble.
The use of methyliminodiacetic acid, described in U.S. Pat. No. 4,294,914 by John Fyson and further described in patent application Ser. No. 08/125,491, by Foster, et al., now U.S. Pat. No. 5,334,491 provides a bleaching solution that accomplishes bleaching at lower molar levels of iron than current biodegradable bleaches and in combination with non-ammonium salts. These features enhance the environmental friendliness of the solution.
But bleaches prepared with tridentate or tetradentate ligands, such as methyliminodiacetic acid, nitriolotriacetic acid, iminodiacetic acid, and .beta.-alaninediacetic acid, among others, do not form as stable of a complex with iron as do hexadentate ligands such as ethylenediaminetetraacetic acid and 1,3-propylenediaminetetraacetic acid, especially in the higher pH wash and/or fixing solutions that follow the bleaching step. These less stable iron complexes, when carried over into a fixer and/or wash, cause an increase in the D-min of the photographic element (especially yellow) due to iron retained in the emulsion. This problem is exacerbated when non-ammonium salts are used in the fixing solution.
Reduction in iron retention can be accomplished to a certain degree by adding a large molar excess of iron ligand to the bleach, which eventually gets `carried over` into the following solutions. This method, however, requires much more of the chelant in the bleach solution and causes losses in bleaching efficiency. It also adds to the effluent load.
U.S. Pat. No. 4,444,873 describes using polycarboxylic acids in a fix to prevent staining in photographic elements which have been processed with a low pH bleach containing a ferric salt, organic acid and halide. It does not address the problem of staining caused by the stronger complexing ferric aminopolycarboxylic acid complexes.
U.S. Pat. No. 4,537,856 describes using water soluble chelates of certain metals in a stabilizer to reduce staining in photographic elements which have been desilvered and stabilized with no washing step. Such metal chelates, however, are not biodegradable.
Therefore, what is needed is a processing solution which, when utilized with a bleaching solution containing a bleaching agent which is a complex of ferric ion and a tridentate or tetradentate ligand, will prevent the formation of iron stains in the photographic element and will be environmentally friendly.